I will have a run of 1" schedule 40 from a sub panel to an outdoor location where I would like to install both an outlet and a light fixture. Must I pull two circuits? May I in that size of conduit?

Some additional notes.

I am putting up some motion-activated (and daylight-deactivated) security lights. The conduit to the fixture just so happens to pass close to a deck that, currently has a non-GFCI outlet and which is probably wired without a ground. So I want to decommission that outlet (covering with a plate, of course) and put a new, grounded, outlet. The maximum I'd have on one circuit here would be three LED security fixtures and one outlet. I have plenty of room in my brand new subpanel for circuits, so it if were code, I'd certainly do this as two circuits. I'm really not that concerned about a fault in one disabling the other until I can repair it.

The conduit isn't in place at all yet; I specified 1" because I used that to have extra room in the underground conduits recently installed, and I dind't think about it very hard. If something smaller meets requirements for fill for two circuits, I'll be happy to buy smaller schedule 40. I'd rather not go down the MWBC path.

I absolutely know to use individual wires, not NM or something, having been taught that by the inspector on a prior effort.

  • Why 1” that is massive for most branch circuits ?
    – Ed Beal
    Mar 10, 2022 at 19:35
  • @EdBeal The OP may clarify, but I assumed it was installed for another reason and repurposed. Mar 10, 2022 at 19:42
  • Will the outlet or light fixture be switched?
    – JACK
    Mar 10, 2022 at 19:44
  • While not required, if you're running new circuits you should consider putting the outdoor lighting, which will probably be LED lighting requiring much less than 1A, onto some other existing lighting circuit so that if a large outdoor load or ground fault blows the new circuit you're not left in the dark. I also recommend putting an indoor Smart dimmer on new outdoor lighting, so you can control it from your phone from anywhere inside or outside the house. Great to be able to light up the garden from your bedroom if you want to see something.
    – jay613
    Mar 10, 2022 at 20:19
  • @nosparksplease, if electrical conduit that is quite large. If they are planning on pulling Romex or other reasons if identified now we may save the OP both time and $ 1” could handle 25 thhn conductors. Maybe it’s there maybe they are putting it in.
    – Ed Beal
    Mar 10, 2022 at 20:24

2 Answers 2


You are not required to put them on separate circuits.

Size of conduit technically is wire size dependent. Minimum size wire for 20A circuits is #12 AWG, which even if separate circuits (5 wires) THWN wire would only require and fit in 1/2" conduit. If you want to create a MWBC that shares a neutral to reduce to four wires then the circuits will have to be fed by adjacent breaker spaces with tied handles or a two-pole breaker.

Often it is recommended to upsize wire for circuits greater than 100', but since long extension cords are often needed outdoors you may want to consider consider upsizing the wire one size anyway, which #10 would require a MWBC to reduce to 4 wires to fit in half inch sch80, 5 #10's wires would still fit in sch40.

  • Outlet and light will need to be GFCI protected
    – JACK
    Mar 10, 2022 at 19:44
  • 1
    @JACK Receptacle will be required to be GFCI protected, lighting is not normally required to be protected. Mar 10, 2022 at 19:47
  • I wasn't really sure about the lighting or exactly what type of fixture or where it might be installed so I threw it out there.
    – JACK
    Mar 11, 2022 at 12:54

You can put up to four small circuits in a conduit pipe. (15-20A).

Larger than that, you get into some issues with thermal derate per NEC 310.15(B)(3)(a), but that's not an issue for four 15-20A circuits or three circuits up to 30A.

Using cable in conduit is problematic, especially with narrow and flat UF cable (NM is not allowed outdoors). There are conduit fill rules, and a cable must be treated same as a round wire of the large dimension. 2 wires in a conduit (or 2 cables) require a conduit inside diameter of 260% of the width of the cable. So even two UF cables may be a problem in 1" conduit.

Assuming the conduit run is continuous between junction boxes, it's best to use THWN or XHHW individual wires inside the conduit. They are much more flexible than cable, and "stranded wire" is even moreso (but hard to put on a screw terminal; wire-nuts just fine). The two circuits can share 1 ground wire. Pulling five stranded THWN wires will be effortless, almost unfairly easy. Inside the junction boxes, the wires can be spliced to UF or NM (indoors) cables for going onward.

Mark the wires with tape so you can distinguish the 2 circuits from each other (particularly, don't mix up their neutral wires!!) This marking is a code requirement. You can just use colored electrical tape to mark wires to match up hot-neutral pairs (so you can tell which circuit is which). If you're familiar with cable, marking white wires has special meaning; that doesn't apply with individual wires. You are also allowed to use the full spectrum of wire colors offered by your supplier (often cheapest to buy "by-the-foot"). So for instance you might get black, red, white x 2 and green, and mark one white wire with black tape and the other with red tape. That's good enough.

THWN wire is sold as "THHN" wire. Almost all wire production is dual-listed THHN and THWN-2. (W for "wet", -2 for 90C thermal rating).

  • A guy I know used indoor NM cable in buried pvc conduit to power a light and receptacle on his front yard mailbox at the street. He glued the joints with the cable inside. When he was telling me about this he said he did it because it was much cheaper than THHN/THWN. He didn't know this was contrary to code. What will happen to the cable over time? If he wanted to pull out the NM cable, could he flood the conduit with soapy water to lubricate? Mar 10, 2022 at 23:44
  • @JimStewart The NM I pulled out of PVC had many small tears in the jacket, probably from pulling friction across joints near bends. Those tears allowed the paper to wick water into the j-boxes at the end of the NM jacket. We don't know the permeability of the wire insulation used in for NM, the specs call it 90°, but unlike MC the cable does not bear a tag giving wire type. Soapy water would be pointless, if jacket is damaged or has become wet it is only good for recycling. Mar 11, 2022 at 0:35
  • 1
    @Jim most likely your friend paid no attention to the idea of making the conduit pullable (having all bends be sweeps and access points in all the right places). As such the wire probably cannot be pulled out at all, and it cannot be repaired except by digging it all up. Very likely the PVC cement degraded the sheath, and water penetrated the sheath, soaked the paper packing, and has degraded the wire insulation. I bet a GFCI at the source won't hold. Mar 11, 2022 at 3:14
  • 1
    @bmargulies Sorry, I cleaned that language up. I'm saying with multiple circuits in THHN you have to define the wire grouping (which hot with which neutral) and you can just use colored electrical tape to do so - there is no restriction on putting colored tape on white wires. (which has a special meaning in cable). Mar 11, 2022 at 3:16
  • 1
    @JimStewart That should be a whole new question, not a sidebar in the comments.
    – FreeMan
    Mar 11, 2022 at 14:20

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